Nutriberries are actually equivalent to pellets - the ingredients just aren't ground up as fine. They do allow for some picking and choosing, but the "glue" that holds them together is loaded with vitamins so it's still superior to plain seed.
You have to be very careful when you're converting a bird to a new diet, because they could starve if they don't accept the new diet right away. It's not stubbornness, they simply don't realize that the new diet is food.
Pellets are very beneficial but the burning question is what percentage of the diet they should be. The pellet companies want you to feed 80-100% pellets but they obviously have a financial interest in having you feed as much as possible. There are some very respectable sources who think that about 50% is the best amount. You're not likely to have nutritional deficiencies at that level, while the bird still gets the psychological benefits of eating a varied diet.
It would be beneficial to read the first part of my pellet article at
http://www.littlefeatheredbuddies.com/i ... llets.html You don't need to read the last part where I dispose of anti-pellet myths unless you want to. There are some people who are strongly anti-pellet and go around telling others that pellets are bad for birds and you shouldn't use them, but this simply isn't true.
If Rio doesn't accept pellets easily you can read the diet conversion article too:
http://www.littlefeatheredbuddies.com/i ... rsion.html He's still young so it might not be too difficult to get him to accept pellets. It's much harder with an older bird. You can always give him lots of Nutriberries while you work on getting him to accept the more conventional pellets.